1. Field of the Invention
The present invention relates to an organometallic complex and an organic electroluminescent device using the same. More particularly, the present invention relates to an organometallic complex capable of emitting light in a red wavelength range, and an organic electroluminescent device using the organometallic complex as an organic layer forming material.
2. Description of the Related Art
Organic electroluminescent (EL) devices are active emission display devices that emit light by recombination of electrons and holes in a thin layer (hereinafter, referred to as “organic layer”) formed of a fluorescent or phosphorescent organic compound when a current is applied to the organic layer. Organic EL devices have advantages such as lightness, simple constitutional elements, easy fabrication process, superior image quality, and wide viewing angles. In addition, the organic EL devices can achieve high color purity and perfectly create dynamic images, and have electrical properties suitable for use in portable electronic equipment due to low power consumption and low driving voltages.
Generally, organic EL devices have a sequentially stacked structure of an anode, a hole transport layer, an emitting layer, an electron transport layer, and a cathode on a substrate. Here, the hole transport layer, the emitting layer, and the electron transport layer are organic layers formed of organic compounds. Organic EL devices having the above-described structure are operated as follows. When voltages are applied to an anode and a cathode, holes from the anode are moved to an emitting layer via a hole transport layer. On the other hand, electrons from the cathode are moved to the emitting layer via an electron transport layer. In the emitting layer, the carriers are recombined to generate excitons. By the radiative decay of the excitons, light emission occurs at the wavelength corresponding to the bandgap of a material.
Materials that can be used to form an emitting layer in an organic EL device are divided into fluorescent materials using a singlet exciton and phosphorescent materials using a triplet exciton according to emission mechanisms. The emitting layer is formed of a fluorescent or phosphorescent material alone or an appropriate host material doped with the fluorescent or phosphorescent material. Singlet excitons and triplet excitons are formed in the host during electronic excitation. At this time, a statistical ratio of the singlet excitons to the triplet excitons is 1 to 3 [Baldo, et al., Phys. Rev. B, 1999, 60, 14422].
An organic EL device including an emitting layer formed of a fluorescent material has a disadvantage in that triplet excitons formed in the host are wasted. On the other hand, an organic EL device including an emitting layer formed of a phosphorescent material has an advantage of 100% internal quantum efficiency since both singlet excitons and triplet excitons can be utilized [Baldo, et al., Nature, Vol. 395, 151-154, 1998]. In this respect, an emitting layer formed of a phosphorescent material can achieve significantly high emission efficiency, relative to an emitting layer formed of a fluorescent material.
When a heavy metal such as Ir, Pt, Rh, and Pd is introduced into an organic molecule, the heavy atom effect leads to spin-orbital coupling, whereby a triplet state and a singlet state are mixed. Therefore, a forbidden transition is induced, which allows efficient phosphorescent emission even at room temperature.
As a high-efficient phosphorescent material, there have been reported various materials based on transition metal compounds containing transition metals such as iridium and platinum. However, phosphorescent materials suitable for highly efficient full color display devices are still being required.